1. Field of the Invention
The present invention relates to a surface corona reactor and to the use thereof. The reactor may be used for oxidative destruction of organic contaminants in air and water and for selective or complete oxidation of organic compounds in the gas and liquid phases in large scale industrial and environmental applications.
2. The Prior Art
Advanced oxidation technologies (AOTs) have been considered for treatment of contaminated water and air as an alternative to thermal destruction and adsorption methods. These are near ambient temperature processes utilizing the hydroxyl radical (.OH) as a primary oxidant. The generation of .OH radicals is commonly accelerated by ozone (O3), hydrogen peroxide (H2O2), titanium dioxide (TiO2), UV radiation, ultrasound, or high electron beam irradiation. Ozone is a reactive gas that has a low solubility in water. It is usually generated on-site from an oxygen source, such as dry air or pure oxygen, by high-voltage surface corona discharge, by ultraviolet radiation or by electrolytic and/or chemical reactions. Ozone is not only a powerful oxidizing agent but also a very powerful non-chemical disinfectant. Ozone has the unique feature of decomposing to a harmless, nontoxic, environmentally safe material, i.e., oxygen. Ozone is currently used for many purposes: taste and odor removal, turbidity reduction, organics removal, microflocculation and manganese oxidation, bacterial disinfections and viral inactivation. However, the ability of ozone to effectively treat wastewater is dependent on the nature of the contaminant. For example, ozone will readily remove color from a dye solution but has much more difficulty reducing the biochemical oxygen demand (BOD) of some organic streams. These differences in ozone effectiveness are due to the chemistry involved in the ozone induced oxidation process. Ozonation techniques, therefore, generally result in partial oxidation of organic pollutants. Other agents such as ultraviolet radiation, oxidants (ozone, hydrogen peroxide) and high pH in homogeneous systems or heterogeneous photocatalytic systems that combine near ultraviolet light (320 to 390 nm) and a light-activated catalyst, such as titanium dioxide, are also used.
Photocatalytic oxidation (PCO) is an alternative technology for cleaning air by removal of volatile organic compounds (VOCs). The technology uses a solid semiconductor photocatalyst—commonly titanium dioxide (TiO2)—that, when illuminated with ultraviolet (UV) light, can promote oxidation of organics at room temperature. This same oxidation of VOCs would require high temperatures (up to 1000° C.) to achieve thermocatalytically. The use of titanium dioxide as a photocatalyst has demonstrated utility in air and water purification, in the capture of sulfur from vapor phase emissions and toxic metal species in combustion exhaust streams, in removal of contaminants from water including methyl tert-butyl ether (MTBE), and in alternative synthesis of partial oxygenates. There remain, however, developmental challenges (problems) to be overcome before heterogeneous photocatalysis can be widely used in large scale processes. These problems include a) the relatively low quantum efficiencies of the catalyst, b) the requirement of near ultraviolet light energy (λ<380 nm) for activation, and c) the inability to construct photocatalytic reactors wherein light distribution is effective and incident on the particle surfaces as required for designing larger scale reactors.
Many studies have been directed toward establishing the relationship between solid-state characteristics and physical characteristics and the photoactivity of the titanium dioxide. The synthesis route is a critical factor in controlling the characteristics of the particulate titanium dioxide product, and its photoactivity. Aerosol processes have proven to be viable routes for the synthesis of nanostructured, pristine and metal doped titanium dioxide particles. Such processes have also been used to deposit titanium dioxide films of varying thickness for different applications. Titanium dioxide films have been demonstrated to be useful in solar cell applications, for the protection of wood, antifog/self-cleaning glass and protection of steel against corrosion. Of all the different methods used for deposition and coating, the flame atmospheric pressure processes, wherein the coating can be produced in a single step, is preferred. Furthermore, flame aerosol coating methods can be readily scaled up to coat large areas.
The geometry of the photocatalytic reactor is also an important factor with respect to the distribution of the light so that it is incident on the titanium dioxide surface. Several different designs have been tested and the results reported in the literature. However, the current technology suffers lack of uniform illumination of the catalyst, inefficient photon utilization, the high cost of energy use, and lack of potential for scaling-up. Nanostructured fixed film reactors have also been demonstrated to be viable for partial oxidation applications. Falling film designs have been demonstrated to be effective in the degradation of MTBE in groundwater samples. Comparative studies have shown that installation costs of conventional photocatalytic reactors are 10 times greater, and annual costs are seven times more than those of granular activated carbon for removing organic compounds from air.
While surface coronas have been generated in electrostatic precipitator type configurations in cylindrical tubular flow reactors, the reactors are not desirably compact.
Surface corona is an electrical discharge (frequently luminous, non-thermal plasma) at the surface of a conductor or between two conductors of the same transmission line, accompanied by ionization of the surrounding atmosphere and often by a power loss. Surface corona discharge technology is similar to the natural process of ozone production via lightning. It occurs when the electric field around the conductor exceeds the value required to ionize the gas, but is sufficient to cause a spark discharge, frequently luminous. Using a surface corona discharge system, ozone is produced by passing air or oxygen through a high voltage electrical discharge, e.g. a surface corona. A minimum of approximately 5,000 volts of electricity is necessary to create the surface corona (14,000 is a practical design maximum voltage). Oxygen in air (containing 21% oxygen) or concentrated oxygen (95% pure oxygen) dried to a minimum of −60° C. (−76° F.) dew point, when passed through the surface corona, has its O2 bond split, freeing two oxygen atoms which then collide with other oxygen molecules to create ozone (O3).
Surface corona generates lower energy electrons (10-20 eV) as compared to the electron beam discharges which produce very high-energy (keV-MeV) electrons. These low energy electrons are accelerated from a very low level of kinetic energy along the high voltage surface corona region and eventually collide with a gas molecule and lose energy by excitation, ionization, dissociation or attachment. After transferring energy to the gas molecule, the low energy electrons are re-energized by the electrical field.
A surface corona discharge also produces a low power UV light on the order of ˜2.0 W, in contrast to the high power UV light obtained from a UV source (1000 kW). Yan et al. have shown that surface corona induced non-thermal plasma can be produced by using pulsed streamer surface corona or by dielectric barrier discharge (J. Electrostatics 44, 17 (1998); J. Electrostatics 51-52, 218, 2001). Surface corona discharges have a number of useful applications. For example, they are used in ozone generators, photocopying machines and electrostatic precipitators. Dielectric barrier discharge driven by an AC power supply has been widely used in the ozone industry. In practice, ozone concentrations of 1-2% using air, and 3-8% using oxygen can be obtained by surface corona discharge generators. Most of the applications so far, such as disclosed by Grymonpre et al., (Chem. Eng. Sci, 54, 3095, 1999; Chem. Eng. Journal 82, 189, 2001, Chem. Eng. Sci. 56, 1035, 2001), have employed an aqueous-phase pulsed streamer surface corona reactor. The dry dielectric barrier discharge based surface corona has been mainly used for the generation of ozone. Researchers such as Futamura, et al. J. Electrostatics 42, 51, 1997; E. M. van Veldhuizen et al. Plasma Chem. Plasma Processing 16, 227, 1996; Vacuum 59, 228, 2000, J. Electrostatics 51-52, 8, 2001, and B. S. Rajanikanth, S. Rout, Fuel Process, Technol. 74, 177, 2001, have shown that surface corona reactors can be used as the primary treatment for the purification of air and water, as well as the treatment of exhaust gas for the decomposition of VOCs and removal of SO2 and NOx.
However, this technology has not been used or explored in the oxidative transformation of organic compounds to value-added products and intermediates. Oxidation of alcohols to aldehydes, ketones or carboxylic acids is one of the most desirable chemical transformations in organic synthesis as these products are important precursors and intermediates for many drugs, vitamins and fragrances. Oxyfunctionalization of hydrocarbons as shown by Barton et al., J. Chem. Soc. Chem. Commun. 731, 1983; J. M. Thomas, Nature 314, 669, 1985; and Ito et al., Nature 314, 721 1985. Such oxidation reactions are widely used in the chemical industry due to the wide ranging utility of the ensuing functionalized compounds as raw materials and intermediates in industrial and pharmaceutical chemistry. As reported for example by R. A. Sheldon et al. Catal. Today 57, 157 2000; P. Griffith, J. M. Joliffe, Dioxygen Activation and Homogeneous Catalytic Oxidation, Simandi, L. L., Ed. Elsevier, Amsterdam, 1991, the industry has developed numerous methods for oxidation of alcohols and hydrocarbons. However, the primary processes for these oxidative transformations still employ toxic, corrosive and expensive oxidants such as chromium (VI) and manganese complexes, stringent conditions like high pressure and/or temperature and use of strong mineral acids as reported by R. A. Sheldon, J. K. Kochi, Metal-Catalyzed Oxidation of Organic Compound, Academic Press, New York (1981) and W. P. Griffith, J. M. Joliffe, Dioxygen Activation and Homogeneous Catalytic Oxidation (Simandi, L. L., Ed). Elsevier, Amsterdam (1991). Some of the methods developed by Murahashi et al. J. Org. Chem. 58, 7328 1993, Inokuchi et al. Tetrahedron Lett 36, 3223, 1995, Iwahama et al., Tetrahedron Lett, 36, 6923, 1995, use O2 in presence of at least a stoichiometric amount of a reactive aldehyde, which form the peracid as the actual oxidizing agent.
There are many reports on effective aerobic oxidation methods that use copper (P. Capdevielle, J. Chem. Res. 10, 1993, Munakata et al. J. Chem. Soc., Chem. Commun., 219, 1980, Senmelhack et al. J. Am. Chem. Soc. 106, 3374, 1984, Marko et al., Science 274, 2044, 1996), palladium, Pd (Marko et al., Science 274, 2044, 1996, Mallat et al., Catal, Today 19, 247, 1994, Brink et al. Science 287, 1636, 2000) and ruthenium compounds (Jensen, J. S. Pugsley, M. S. Signam, J. Am. Chem. Soc. 123, 7475, 2001. Cornelis, Synthesis 909, 1985; Cseri et al., Bull. Soc. Chim. Fr. 133, 547, 1996; Heravi et al., Chem. Commun, 833, 1999; Narayanan, Appl. Catal. A. Gen. 199, 1, 2000) and using photocatalysis (Pillai, E. Sable-Demessie, J. Catal, 211, 434, 2002) Some of these methods are limited to benzylic alcohols and often require two equivalents of the catalyst per equivalent of the alcohol. Senmelhack, C. R. Schmid, D. A. Cortes, and C. S. Chon, J. Am. Chem. Soc. 106, 3374, 1984 showed that the presence of a base and additives like di (t-butyl azodihydrazine) require or involve a complex catalyst preparation that is difficult to recycle. In various studies (Pillai, E. Sahle-Demessie, J. Catal. 211, 434 2002; Parvulescu, et al., J Mol. Catal. A; Chem. 140, 91, 1999; Spinace, et al. J. Catal. 157, 631, 1995; Zahedi-Niaki, et al. J. Catal. 177, 231, 1998) hydrocarbon oxidations have been used in a homogeneous and heterogeneous catalytic systems employing different oxidants such as hydrogen peroxide, t-butyl hydroperoxide and molecular oxygen over various catalysts such as Na—GeX zeolite, TS-1 and Ti-MCM-41 and metal containing AIPO redox molecular sieves. The present inventors have recently reported effective hydrocarbon oxidations over vanadium phosphorus oxide catalysts using hydrogen peroxide (U. R. Pillai, E. Sahle-Demessie, Chem. Commun. 2142, 2002; New J. Chem. 27, 525, 2003). Although such processes are currently being utilized they have low energy efficiencies and generate environmentally hazardous waste and by-products. The increased environmental concerns in the recent years call for use of environmentally benign oxidants like molecular oxygen or hydrogen peroxide, rather than organic peroxide and stoichiometric metal oxides, which have been widely employed until now. Hydrogen peroxide oxidation, however, is relatively less economical due to its cost and relatively poor efficiency. In industrial chemistry, heterogeneous catalyst systems are preferred over homogeneous systems due to ease in separating and recycling. Therefore, there is a continuing demand for a more efficient, cost effective and environmentally friendly process for the oxidation of alcohols and hydrocarbons.